This invention relates to communication stations, such as radio communication transceivers, employing a "push-to-talk" switch. A radio transceiver capable of transmitting and receiving and having a push-to-talk switch, usually physically associated with a microphone element or near the operator's hands, normally operates in the receive mode. To transmit, an operator presses the push-to-talk switch. Pressing the push-to-talk switch usually operates a relay within the transceiver that causes it to operate in the transmit mode. When the operator releases the push-to-talk switch, the transceiver reverts to receive mode operation.
In communication networks wherein a plurality of such transceivers communicate with one another over a limited number of communication channels, any one transceiver that continuously transmits may tie up the network and prevent others from communicating thereon. Such continuous transmission by a communication station within a public service or safety network such as police, fire, airline control, and utilities, can create a safety hazard.
It is possible for such continuous transmission to occur without the knowledge of the communication station operator. This can occur when the push-to-talk switch, becomes stuck.
In an airline communication network, this can and does occur on occasion when pilots utilize a communication radio having a handheld microphone with a push-to-talk switch mounted directly on the microphone. Such a microphone, laid upon the seat, within reach of the pilot, can slip between the seats and become lodged between the seats or mechanical equipment and the push-to-talk switch may become pressed without knowledge of the pilot. If the radio is squelched to eliminate the "drone" of background noise when operating in the receive mode, the pilot can transmit without realizing that transmission is occurring. If his signal is sufficiently powerful, he can, in effect, jam communications on the channel on which he is transmitting and prevent others from communicating important messages including clearances, runway approaches, wind velocities and the like.
One approach to obviating the safety problem associated with a stuck push-to-talk switch is to provide means for limiting the time permitted for each transmission i.e. inhibit further transmission after a predetermined period of time has lapsed. One embodiment of this approach is shown in U.S. Pat. No. 3,500,459--Battin et al., issued Mar. 10, 1970. In the Battin arrangement, a timer is provided for establishing a predetermined maximum period of time for each transmission. The pressing of the push-to-talk switch, in effect, sets the timer which begins to count time during the transmission. When the timer times out, if the push-to-talk switch has not been released, means, coupled to the timer, electrically disengage a relay associated with the push-to-talk switch. Thus, the radio station is "forced" back into the receive mode, even with the push-to-talk switch continuously pressed. Thus, in effect, the push-to-talk switch is defeated. The difficulty associated with this approach is that every transmission must be limited to a predetermined interval of time. In the event that a more lengthy communication is required, the operator must periodically re-key his push to talk switch. During an emergency wherein the operator's attention must be directed fully to the emergency and the substance of communication, the required re-keying of the push-to-talk switch provides a distraction that, in and of itself, contributes to the emergency by diverting the operator's full attention.